1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the same. More particularly, the present invention relates to a device suitably used for an image forming apparatus, such as a laser beam printer or a digital copying machine having an electrophotographic process, in which at least one light beam emitted from a light source means is deflected by a deflection element and then optically scans a scanning surface through an image forming element having an fxcex8 characteristic, thereby recording image information.
2. Related Background Art
In a conventional scanning optical apparatus used for a laser beam printer (LBP), a digital copying machine, or the like, the light beam optically modulated according to an image signal and emitted from a light source means is regularly deflected by a light deflector composed of a rotary polygon mirror (polygon mirror). The deflected light beam is converged to form a spot on a surface of a photosensitive recording medium (photosensitive drum) by a scanning optical element having an fxcex8 characteristic, and optically scans the surface of the photosensitive drum to record an image thereon.
FIG. 5 is a schematic diagram that shows the main part of a conventional scanning optical apparatus of this type.
In this figure, diverging light beam emitted from a light source means 51 is converted into a substantially parallel light beam by a collimator lens 52, an aperture stop 53 limits the parallel light beam (quantity of light), and the limited parallel light beam strikes a cylinder lens (cylindrical lens) 54 having a predetermined refracting power only in the sub-scanning direction. The substantially parallel light beam striking the cylinder lens 54 is emitted as it is (in the state of substantially parallel rays) in a main scanning cross-section. In a sub-scanning cross-section, the light beam is converged to form a nearly linear image on a deflection surface (a reflection surface) 55a of a light deflector 55 composed of a rotary polygon mirror (polygon mirror).
The light beam reflected and deflected by the deflection surface 55a of the light deflector 55 is guided by an image formation means (fxcex8 lens) 56 having an fxcex8 characteristic to strike a photosensitive drum surface 58 to be scanned. By having the light deflector 55 rotate in the direction of an arrow A, the photosensitive drum surface 58 is optically scanned in the direction of an arrow B. In this manner, an image is recorded on the photosensitive drum surface 58 that is the recording medium.
In recent years, various scanning optical apparatuses (multi-beam scanning optical apparatuses) have been proposed in response to the increasing demand for high-speed and high-resolution LBPs. Each of such scanning optical apparatuses adopts a multi-beam laser that uses a plurality of laser light sources (light-emitting units) as its light source. With this construction, the scanning optical apparatus simultaneously scans surfaces of a plurality of recording media by irradiating the surfaces with a plurality of light beams.
In addition to high-speed and high-resolution image forming apparatuses having the electrophotographic process, various color image forming apparatuses that support high-speed color image formation have also been proposed. FIG. 6 is a schematic diagram that shows the main part of a tandem type color image forming apparatus that simultaneously uses a plurality of scanning optical apparatuses described above to record image information in various colors on the surfaces of a plurality of different photosensitive drums.
In the color image forming apparatus shown in this drawing, four scanning optical apparatuses (61, 62, 63, and 64) having the construction shown in FIG. 5 are arranged so as to respectively correspond to four colors: C (cyan), M (magenta), Y (yellow), and B (black). These scanning optical apparatuses record image signals on the surfaces of photosensitive drums (71, 72, 73, and 74) in parallel. With this construction, the color image forming apparatus prints color images at high speed.
One crucial aspect of the scanning optical apparatuses used for such a color image forming apparatus is the productivity and cost. Therefore, in usual cases, the scanning optical apparatuses are provided with image formation means (fxcex8 lenses) that have been produced at low cost by molding of plastic.
In the scanning optical apparatus that uses a plastic lens, deviations in an image forming position are caused by changes in a focus position and chromatic aberration of magnification that are caused by various factors. Three major factors are given below.
Deviation in initial wavelength between a plurality of laser light sources
Deviation in wavelength caused by mode hopping of a semiconductor laser due to environmental variations
Fluctuation in refractive index of the plastic lens due to environmental variations
The deviations in the focus position result in the enlargements of a beam spot and degrade image quality. Deviations in the image forming position due to chromatic aberration of magnification cause magnification changes of recorded images. In particular, in a tandem type color image forming apparatus, the differences in magnification change among a plurality of scanning optical apparatuses cause registration deviations of each color (chromatic deviations), which results in degraded image quality.
A color image forming apparatus that solves this problem is proposed, for instance, in Japanese Patent Application Laid-open No. 11-223784. This color image forming apparatus corrects chromatic aberration of magnification and compensates for the temperature at a focus position using an image formation means constructed by combining a refractive optical element with a diffraction optical element.
The following is a description of an ordinary color image forming apparatus that includes an image formation means constructed by combining a refractive optical element with a diffraction optical element.
(a-1) The power of the diffraction optical element which is disposed at a far side with respect to the optical deflector needs to be increased to reduce the magnification (the sub-scanning magnification) of the image formation means in the sub-scanning direction as necessary. It is preferred to set the power ratio between the refractive optical element and the diffraction optical element at a desired ratio for an aberration correction purpose. It is difficult, however, to produce a high-power diffraction optical element because a grating pitch of its diffraction grating is reduced according to the increase in the power of the diffraction optical element. When the sub-scanning magnification is large, the scanning line deviates from a desired position due to various factors, such as the errors caused during the processing of deflection surfaces of a polygon mirror and the vibrations of the apparatus itself. This tends to cause jitter, which is a critical design factor because jitter causes registration deviations of each color (chromatic deviations) in a color image forming apparatus.
(a-2) When a scanning optical apparatus is manufactured and assembled using a plurality of optical elements (a refractive optical element and a diffraction optical element, in this case), if the refractive optical element and the diffraction optical element are eccentrically arranged due to manufacturing errors, the traveling path of a laser spot (so-called scanning line) is bent in the scanning optical apparatus. For instance, in the conventional case shown in FIG. 5, if a toric lens 56a is arranged at a position that is shifted 0.1 mm in a direction perpendicular to the plane of paper at normal mechanical accuracy, the bend degree of the scanning line becomes about 29 xcexcm. In particular, in a tandem type color image forming apparatus, the differences in bend degree of scanning line among scanning optical apparatuses corresponding to respective colors cause registration deviations of each color (chromatic deviations). As a result, image quality is degraded.
An object of the present invention is therefore to provide a scanning optical apparatus where a diffraction optical element that is a component of an image formation means has an appropriate shape in the sub-scanning cross-section and the sub-scanning magnification of the image formation means is reduced, thereby improving the performance, such as jitter and image quality, of the scanning optical apparatus, and to provide an image forming apparatus using the scanning optical apparatus.
Another object of the present invention is further to provide a scanning optical apparatus where a refractive optical element is arranged at or near a front focus position of a diffraction optical element in the sub-scanning cross-section, thereby reducing the bend degree of a scanning line, and to provide an image forming apparatus using the scanning optical apparatus.
According to a first aspect of the present invention, there is provided a scanning optical apparatus comprising:
incident optical means for causing at least one light beam emitted from light source means to be incident on deflection means; and
image formation means including at least one refractive optical element and at least one diffraction optical element for imaging the at least one light beam reflected and deflected by the deflection means on a surface to be scanned,
in which the diffraction optical element has at least one of an incident surface having a convex shape in a sub-scanning cross-section facing the deflection means and an exit surface having a convex shape in the sub-scanning cross-section facing the surface to be scanned.
According to a second aspect of the present invention, in the first aspect of the invention, a diffraction grating is formed on one of an incident surface and an exit surface of each diffraction optical element.
According to a third aspect of the present invention, in the first aspect of the invention, the image formation means comprises a single refractive optical element and a single diffraction optical element and satisfies:
xe2x80x830.5 less than |e2/s1|
where e2 is a distance between an exit surface of the refractive optical element on an optical axis and the incident surface of the diffraction optical element on the optical axis, and
s1 is a distance between the incident surface of the diffraction optical element on the optical axis and a front focus position of the diffraction optical element in the sub-scanning cross-section.
According to a fourth aspect of the present invention, in the first aspect of the invention, the refractive optical element has a meniscus shape in a main scanning cross-section such that a concave surface faces the deflection means side.
According to a fifth aspect of the present invention, in the first aspect of the invention, a front focus position of the diffraction optical element in the sub-scanning cross-section is provided between a power arrangement in the sub-scanning cross-section of an on-axis refractive optical element and a power arrangement in the sub-scanning cross-section of an off-axis refractive optical element, in an optical axis direction.
According to a sixth aspect of the present invention, in the first aspect of the invention, the apparatus further comprises:
at least one of tilt adjusting means and shift adjusting means for adjusting a position of the diffraction optical element.
According to a seventh aspect of the present invention, there is provided an image forming apparatus comprising:
a scanning optical apparatus described in any one of the first to sixth aspects of the invention;
a photosensitive member arranged on the surface to be scanned;
a developing member for developing an electrostatic latent image formed on the photosensitive member by the light beam scanned by the scanning optical apparatus as a toner image;
a transfer member for transferring the developed toner image onto a material to be transferred;
a fixing member for fixing the transferred toner image on the material to be transferred; and
a printer controller for converting code data inputted from an external device into an image signal and inputs the image signal into the scanning optical apparatus.
According to an eighth aspect of the present invention, there is provided an image forming apparatus comprising:
a plurality of scanning optical apparatuses described in any one of the first to sixth aspects of the invention;
in which a color image is formed by guiding a plurality of light beams emitted from the respective scanning optical apparatuses onto a plurality of corresponding image bearing member surfaces, respectively, and scanning the plurality of image bearing member surfaces with the plurality of light beams.
According to a ninth aspect of the present invention, there is provided a scanning optical apparatus comprising:
incident optical means for causing at least one light beam emitted from light source means to be incident on deflection means; and
image formation means including at least one refractive optical element and at least one diffraction optical element for imaging the at least one light beam reflected and deflected by the deflection means on a surface to be scanned,
in which the following condition is satisfied:
0.5 less than |e2/s1| less than 1.2
where e2 represents a distance between an exit surface of the refractive optical element on an optical axis and an incident surface of the diffraction optical element on the optical axis, and
s1 represents a distance between the incident surface of the diffraction optical element on the optical axis and a front focus position of the diffraction optical element in a sub-scanning cross-section.
According to a tenth aspect of the present invention, in the ninth aspect of the invention, the diffraction optical element has at least one of an incident surface having a convex shape in a sub-scanning cross-section facing the deflection means and an exit surface having a convex shape in the sub-scanning cross-section facing the surface to be scanned.
According to an eleventh aspect of the present invention, in the ninth aspect of the invention, a diffraction grating is formed on one of the incident surface and the exit surface of the diffraction optical element.
According to a twelfth aspect of the present invention, in the ninth aspect of the invention, the refractive optical element has a meniscus shape in a main scanning cross-section such that a concave surface faces the deflection means.
According to a thirteenth aspect of the present invention, in the ninth aspect of the invention the front focus position of the diffraction optical element in the sub-scanning cross-section is provided between a power arrangement in the sub-scanning cross-section of an on-axis refractive optical element and a power arrangement in the sub-scanning cross-section of an off-axis refractive optical element, in an optical axis direction.
According to a fourteenth aspect of the present invention, in the ninth aspect of the invention, the apparatus further comprises:
at least one of tilt adjusting means and shift adjusting means for adjusting a position of the diffraction optical element.
According to a fifteenth aspect of the present invention, in the ninth aspect of the invention, the following condition is satisfied:
0.55 less than |e2/s1| less than 1.1
According to a sixteenth aspect of the present invention, there is provided an image forming apparatus comprising:
a scanning optical apparatus described in any one of the ninth to fifteenth aspects of the invention;
a photosensitive member arranged on the surface to be scanned;
a developing member for developing an electrostatic latent image formed on the photosensitive member by the light beam scanned by the scanning optical apparatus as a toner image;
a transfer member for transferring the developed toner image onto a material to be transferred;
a fixing member that fixes the transferred toner image on the material to be transferred; and
a printer controller for converting code data inputted from an external device into an image signal and inputs the image signal into the scanning optical apparatus.
According to a seventeenth aspect of the present invention, there is provided an image forming apparatus comprising:
a plurality of scanning optical apparatuses described in any one of the ninth aspects to fifteenth aspects of the invention;
wherein a color image is formed by guiding a plurality of light beams emitted from the respective scanning optical apparatuses onto a plurality of corresponding image bearing member surfaces, respectively, and scanning the plurality of image bearing member surfaces with the plurality of light beams.